ABSTRACT

The Cumberland Bay Formation, South Georgia, is a deformed Late Jurassic-Early Cretaceous volcaniclastic turbidite formation several kilometers thick. It has been interpreted as the fill of a back-arc basin which was active on the southwest margin of Gondwana during the Mesozoic. The Cumberland Bay Formation was derived entirely from the volcanic arc and was deformed when the basin closed in midCretaceous times.

Sole marks (n = 510), oriented fossil wood (n = 64), and small channels (n = 9) identified the paleocurrent directions, and slump folds (n = 95) indicated the paleoslope. In most areas sole marks show a southeast-northwest trend roughly parallel to the axis of the island; unambiguous current-direction indicators show that the "axial" dispersal was towards the northwest. Along the southwest coast, paleocurrent distributions are bimodal, and there is a "lateral" mode at a high angle to the axial mode in most areas. Beds bearing lateral-mode sole marks are thicker than those with only axial-mode sole marks. Fossil-wood and channel orientations provide further evidence of a major axial dispersal pattern. Slump folds show that over much of the outcrop, paleoslopes were roughly northerly, a though there was a topographic high some 40 km across, on the southwest coast, that may have been a small depositional lobe due to lateral input. This lobe appears not to have inhibited the axial flow.

The overall paleocurrent pattern defines a curve concave to the southwest which parallels the shape of the island; this pattern may reflect original basin shape. The modern Grenada Basin (Lesser Antilles) provides an analogue which can be used to model depositional patterns within the Cumberland Bay Formation.